This invention relates to a method for synthesizing rod-shaped semiconductor nanocrystals, inter alia rod shaped Group III-V semiconductor nanocrystals.
The following references are considered to be pertinent for the purpose of understanding the background of the present invention:
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Miniaturization of electronic and optical devices requires semiconductors of nanometer size domain. Inorganic semiconductors, in particular Group III-V semiconductors, exhibit features that make them attractive for use in solid state electronics as well as optical devices (e.g., high thermal stability, high electron mobility, low energy band gap, and direct-band gap behavior.
Developing preparation methods of semiconductor nanocrystals has been an important branch of synthetic chemistry. For many of the semiconductors, syntheses through the reaction of simple reactants have been proved to be impossible, thus only until recent years, some of them can be prepared through the use of organometallic precursors in organic solvents. The typical examples are Group III-V semiconductor nanocrystals, which are formed by dissolving a Group IIIa precursor and a Group Va precursor in a solvent and then applying heat to the obtained mixture of solvent and precursors. More specifically, Group III-V semiconductor nanocrystals may be produced by using silyl cleavage. Specifically, Group III halides have been reacted with E(SiMe3)3, where E=P,As, in hydrocarbon solvents to yield nanocrystalline III-V semiconductors [1].
Alivisatos et al. [2] described a process for forming Group III-V semiconductor nanocrystals wherein size control is achieved through the use of a crystallite growth terminator such as nitrogen- or phosphorus-containing polar organic solvent, for example pyridine, quinoline or mono-, di-, and tri-(C1-6 alkyl)phosphines.
High quality group III-V semiconductor nanocrystals were produced by a method consisting of injecting the precursors into a hot coordinating solvent such as trioctyl-phosphine (TOP) [3].
All the above-mentioned processes can yield only spherically shaped nanocrystal semiconductors (also termed quantum dots) of the group III-V.
In the nanometer size domain the properties of semiconductors depend not only on size but also on shape. The growth of crystalline wire-like structures based on the vapor-liquid-solid (VLS) mechanism has been developed [4]. In the VLS method, a liquid metal cluster acts as a catalyst where the gas-phase reactants adsorb, subsequently leading to whisker growth from the supersaturated drop. Laser ablation was combined to form nanometer-sized metal clusters as catalysts with the VLS mechanism, to grow a variety of semiconductor nanowires, doped nanowires, and nanowire superlattices with lengths in the micrometer range [5-8]. The diameters of the nanowires could be tuned by changing the size of the catalyst clusters [9] and for silicon nanowires, the smallest diameter reported was 6 nm. For InAs and other III-V semiconductor nanowires, the average diameters are larger than 10 nm. Such diameters are at the onset of the strong quantum confinement regime providing rather limited band gap tunability by size effects. A whole set of optoelectronic devices including transistors, detectors and a light emitting diode were demonstrated using these nanowire building blocks [10, 11]. Micron long silicon nanowires with diameters of 4-5 nm were also prepared in a pressurized solution system by using gold nanocrystals as catalyst [12].
Rod-shaped nanocrystals are interesting because the rod shape produces polarized light emission leading to polarized lasing [13, 14]. Additionally, rod-shaped particles are suitable for integration into nano-electrode structures for the production of electronic devices such as sensors, transistors, detectors, and ligh-temitting diodes [11, 15]. A recent example of rod-shhaped nanocrystals was described in relation with CdSe nanocrystals [16, 17]. Shape control in this case was realized by kinetically controlled growth along the special c-axis of wurtzite CdSe nanocrystals through the use of a mixture of surfactants. Such a process is not applicable for the growth of cubic-structured semiconductor nanorods, e.g. nanorods of a few important III-V semiconductors. Therefore a novel and completely different rod growth mechanism is needed for these compounds.
A process for the formation of shaped Group III-V semiconductor nanocrystals is described in U.S. Pat. No. 6,306,736 [18], according to which a solution of the semiconductor nanocrystal precursors was contacted with a liquid media comprising a binary surfactant mixture capable of promoting the growth of either spherical semiconductor nanocrystals or rod-like semiconductor nanocrystals.
There is a need in the art to facilitate preparation of nanocrystalline semiconductors, especially Group III-V semiconductors, having rod-like shape of controlled dimensions in a controllable and repeatable manner, which are not available to date. The term xe2x80x9cIII-V semiconductorxe2x80x9d is used to describe crystalline material or solid solution formed from the reaction of at least one metal precursor from group IIIa of the Periodic Table of the Elements (B, Al, Ga, In, and Tl) and at least one element from group Va of the Periodic Table of the Elements (N, P, As, Sb, and Bi). It should be noted here that the expression xe2x80x9cat least onexe2x80x9d is used in recognition of the fact that, for example, a particular Group IIIa metal precursor may be reacted with more than one particular Group Va precursor, e.g., GaAsP, and vice versa, e.g. InGaAs.
The term xe2x80x9cnanocrystal having rod-like shapexe2x80x9d or xe2x80x9cnanorodxe2x80x9d is meant a nanocrystal with extended growth along the first axis of the crystal while maintaining the very small dimensions of the other two axes, resulting in the growth of a rod-like shaped semiconductor nanocrystal of very small diameter, where the dimensions along the first axis may range from about 10 nm to about 500 nm. The nanorods prepared by the method of the invention may have one end made of non semiconducting material.
Examples of Group III-V semiconductors that may be prepared by the method of the invention are InAs, GaAs, GaP, GaSb, InP, InSb, AlAs, AlP, AlSb and alloys such as InGaAs, GaAsP, InAsP. Examples of Group II-VI semiconductors that may be prepared by the method of the invention are CdS, CdSe, CdTe, ZnS, ZnSe, ZnTe, HgS, HgSe, HgTe and the like. Examples of Group I-VII semiconductors are CuCl, CuBr, CuI, AgCl, AgBr, AgI and the like.
The main idea of the present invention is based on introducing nanoparticles of a metal catalyst that serve as starting nanocrystals from which nanorods of inorganic semiconductors grow. Without being bound to theory, it is proposed that the reaction precursors dissolve in the metal nanoparticles and the semiconductor nanorods grow from these particles.
The metal catalyst can be for example a noble metal, e.g. gold, a Group IIIb metal, e.g. In, Ga, Al or a transition metal, e.g. Fe, Zn, Cd, etc. The catalyst nanoparticles may either be formed in-situ in the reaction process from one of the precursor materials by using a reducing agent or be added to the reaction process.
Thus, in one aspect the present invention provides a new method for the production of inorganic semiconductor nanocrystals having a rod-like shape, the method comprising:
reacting, in a high-boiling point organic solvent, a two-source precursor solution comprising at least one metal source and at least one nonmetal source, or a precursor solution comprising a single-source precursor, with a metal catalyst or an agent capable of producing said metal catalyst, said high-boiling point organic solvent having a temperature above 200xc2x0 C., thereby forming a reaction product comprising semiconductor nanocrystals of various shape;
cooling the reaction product, and
subsequently exposing said cooled reaction product to at least one centrifugal step so as to obtain semiconductor nanocrystals having substantially rod-like shape.
The rod-shaped nanocrystals obtained by the method of the invention usually have organic ligands as a coating on their outer surfaces. Such organic ligands affect the solubility of the particles and may be substituted or removed, according to the application intended for said particles after the reaction is completed.
The reaction precursors used in the method of the invention are selected from Group Ib, IIb or IIIa metal compounds, Group VIIa, VIa or Va non-metal compounds, Group IV element compounds, compounds comprising both Group Ib and Group VIIa elements, compounds comprising both Group IIb and Group VIa elements and compounds comprising both Group IIIa and Group Va elements. Consequently, the inorganic semiconductor rods prepared by the method of the invention are selected from Group III-V, Group III-V alloys, Group II-VI, Group I-VII, and Group IV semiconductors.
Examples for single-source precursor compounds, e.g. for Group III-V semiconductors: {t-Bu2In[xcexc-P(SiMe3)2]}2, [t-Bu2In(xcexc-PH2)]3; for Group IV semiconductors: (C6H5)2SiH2).
According to a preferred embodiment, the present invention provides a method for the formation of Group III-V semiconductor nanocrystals having rod-like shape, comprising
(i) reacting, in a high-boiling point organic solvent, a precursor solution comprising at least one Group IIIa metal source and at least one Group Va nonmetal source with a metal catalyst or an agent capable of producing said metal catalyst, said high-boiling point organic solvent having a temperature above 200xc2x0 C., thereby forming a reaction product comprising Group III-V nanocrystals of various shape;
(ii) cooling the reaction product, and
(iii) subsequently exposing said cooled reaction product to at least one centrifugal step so as to obtain Group III-V semiconductor nanocrystals having substantially rod-like shape.
Preferably, the metal catalyst is produced in situ by using an agent capable of producing it, for example by using a reducing agent capable of reducing the Group IIIa metal precursor into the corresponding Group IIIa metal.
According to another preferred embodiment, the present invention provides a method for the production of InAs semiconductor nanocrystals having rod-like shape, comprising introducing a precursor solution of an In source and an As source into a high-boiling point organic solvent comprising NaBH4, said high-boiling point organic solvent having a temperature above 200xc2x0 C., thereby forming upon cooling and optionally annealing a reaction product comprising InAs nanocrystals of various shape, and subsequently carrying out at least one centrifugal step so as to obtain InAs semiconductor nanocrystals having rod-like shape.
Semiconductor nanocrystals having rod-like shape and formed by the above-described method present another aspect of the present invention.
Semiconductor nanocrystals are of interest for use in optical displays, optical detectors, data communication systems and biological applications such as fluorescence marking of biomolecules, and as sensors. Rod/shell nanocrystals based on the nanorods prepared by the method of the invention may provide further control of the optical and electronic properties of the nanorods specifically as a method to enhance their fluorescence efficiency that is important for various applications [19, 20].